We have bought a wet room type shower tray 1200x1200 its plumbed into the floorboards and two sources have told us contradictory advice re tanking before tiling. The shop we bought the tray from (Fired Earth) told us we must tank the area up the walls (plaster) and floor surround (plywood) before tiling. A tiler has told us that with ceramic tiles and a suitable adhesive we don't need to. Which is correct? The tiles chosen are a 5cm ceramic mosaic 30 x 30 sheet (wall) and 30 x 30 ceramic floor tile. The floor adhesive will need to be flexible as underfloor heating is going down also. If we need to tank the area anyone got any recomendations?? material...paint on....any reliable brands?